What you describe sounds commonplace for those of us who suffer with allergies. Many allergy sufferers take Zyrtec. I guess that the primary difference would be if you discovered exactly what it is that he is allergic to. This would require allergy testing (usually a skin allergy test) -see Wikepedia for a full explanation of skin allergy testing. Knowing what he is allergic to can be very helpful in attempting to avoid those things in order to prevent a reaction such as the one that you described.
I hope that he gets better and I wish you both the best.
Peace and Blessings